1. Field of the Invention
The present invention relates to improvements in a pull-back type indirect extrusion press such that the scalping of billets can be performed inside the pressing machine independently of extrusion cycles and the scalped billet is allowed to remain in the machine in readiness for an extrusion cycle. The invention also provides means for prevention of cooling and temperature control of said scalped billet and for prevention of surface oxidation of said billet while it is held in the machine.
2. Description of the Prior Art
A pull-back type indirect extrusion press is known which comprises a pair of movable platens opposed to each other on opposite sides of a fixed central platen and connected together by tie rods, and a main cylinder mounted on said fixed platen for moving said movable platens, wherein a billet is placed on the press centerline between one movable platen and said fixed platen and is then charged into a container disposed between the other movable platen and said fixed platen and is extruded through a die stem to provide a required product. In this connection it is to be noted that the indirect extrusion press cannot process black (skin) billets without removal of their skins. Thus, conventionally, they have to be subjected to machining or scalping on a machine tool or scalper separate from the press prior to extrusion. From the standpoint of initial and running costs, it is advantageous to effect the scalping of billets inside the extrusion press. For this reason, there have recently been proposed some press designs having a scalping member built in the press, a typical one of which is adapted so that a billet is inserted into the billet container while being scalped by a scalping member, such as a scalping die, attached to the end opening in the container. This type of press, however, encounters the following problems.
The container is supported on a container holder and movably installed adjacent the fixed platen and, as is known in the art, a shearing member or device for severing discards must be installed in the space defined between the fixed platen and container, thus sometimes necessitating an operation intended to avoid interference with the scalping member. Further, a scalping cycle must of course be added to the extrusion cycle, thus lengthening the latter. As a result, the press mechanism and extrusion process become complex.
Another consideration which must be given to the indirect extrusion press is an taper heating, which is important technique involved in heating billets. The taper heating effect is liable to lessened as the hot billet tends to cool down prior to extrusion and hence maintaining the temperature to the hot billet is of special importance. Moreover, in the press of the type adapted to apply scalping to billets for removal of their skins while the billet is held in the press, the object of the problems of cooling and oxidation of hot billets become more pronounced.